The British patent document 2,013,635 A discloses a closure means of this kind wherein the radial projection rests against the inside wall of the bottle neck in the immediate vicinity of its own orifice rim. The cylindrical sealing element is very short and the outside of the projection lies closely to the bottom of the cap-shaped closure component. The bottom bulges outwardly in case of overpressure. When deforming in this convex manner, the sealing projection follows said mainly axially directed bulging until, at a given overpressure and hence at a given bulge, it is released from in the inner mouth rim of the bottle's neck and in this manner subtends gap through which the overpressure may be relieved. In this manner the said sealing means forms an overpressure valve averting bottle bursting and entailed dangers.
This known closure means incurs a drawback in that in practice the front inner rim at the mouth of the bottle neck demands high precision of manufacture, so that no defined rest of the sealing projection is assured exactly in the critical range wherein the overpressure valve formed by the closure means shall open and close. In addition, as regards such bottles, and especially those glass bottles to be reused several times, the inner rim edge of the bottle neck is damaged and thus will not be sealing.
Another drawback of this known closure means is that on account of pressure changes arising in practice by heating and then cooling the contents of the bottle fitted with said closure means, the sealing projection steadily slides in the axial direction on the inside surface of the mouth of the bottle acting as a valve seat, whereby the sealing surface of the projection may be abraded or damaged and then be leaking.
Lastly the known closure means incurs the drawback that the necessity of axial displacement in turn requires axial yielding by the cap outside the cylindrical sealing element. This requirement is met in this known closure means embodied by a screw cap by shearing the bottom outside the cylindrical sealing element and by axially stretching the outer cylindrical part. However said stretching includes prestressing which in turn depends on the screw-on torque, in any event it depends during customer use on the particular torque applied by the user in screwing on the screw cap. Consequently the pressure at which for safety reasons there shall be pressure relief will not be provided with the desired reliability to preclude bottle bursting at excessive pressure.
The British patent document 958,417 discloses a closure means of a similar kind, also in the form of a screw cap fitted with projections inside its bottom and serving as stops and coming to rest against the rim edge of a bottle neck when said cap is being screwed onto it. In this design the stretching of the outer cap part comprising the thread is not utilized to impart axial displaceability to the bottom. The elasticity of this axial displaceability being determinant for the opening pressure, whereas only the region between the cylindrical sealing element and the outer rim of the top end of the bottle neck are available for yielding, the bottom must be made very thin to achieve adequately elastic yielding. This feature is a drawback, the more so that it entails very tight manufacturing tolerances.
The objective of the invention is to create a closure means of the kind defined in the preamble of claim 1 wherein inaccuracies of or damage to the front inner edge of the mouth of a bottle neck are not disadvantageous and where its sealing will not be degraded even in the presence of frequent pressure changes.